How it works
Pouring a concrete slab or footing starts with knowing how much material to have on hand. Enter the length and width of your pour area in feet, plus the thickness in inches. The calculator gives you the volume in cubic yards — the unit ready-mix suppliers quote — and translates that into a count of 60 lb or 80 lb bagged concrete for smaller jobs.
A typical shed floor, patio, or sidewalk pad calls for a 4-inch pour. Footings under a wall or post usually run 8 to 12 inches. Either way, you need enough mixed concrete on site to fill the forms in one continuous pour, because cold joints between partially cured batches create weak spots. Reinforcement — wire mesh or rebar — does not change the volume calculation, but it does affect staging: you still need the full quantity of concrete to cover the steel and reach the finished grade line.
The formula
Cubic Yards = (Length (ft) × Width (ft) × Thickness (in)) / 324
That constant folds two conversions together: inches to feet (÷ 12), then cubic feet to cubic yards (÷ 27). For bag count, a 60 lb bag of premix yields roughly 0.45 cubic feet of cured concrete; an 80 lb bag yields about 0.60 cubic feet.
Worked example
You are pouring a 10 ft × 10 ft shed slab at 4 inches thick.
Volume = (10 × 10 × 4) / 324 = 400 / 324 = 1.23 cubic yards
Convert that to cubic feet to work out bag count:
Cubic feet = 1.23 × 27 = 33.33 cu ft
Each 60 lb bag yields 0.45 cubic feet, so:
Bags = 33.33 / 0.45 = 74.1 → 75 bags
Add a 5–10 % waste factor for spillage, over-excavation, and an uneven sub-base:
Bags with waste = 75 × 1.10 = 82.5 → 83 bags
For the same slab using 80 lb bags instead:
Bags = 33.33 / 0.60 = 55.6 → 56 bags
Bags with waste = 56 × 1.10 = 61.6 → 62 bags
Tips
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 60 lb | 0.45 cu ft | ~60 bags |
| 80 lb | 0.60 cu ft | ~45 bags |
At around 1.2 cubic yards, this shed slab sits right at the crossover point. Bagged concrete makes sense for pours under roughly 1 cubic yard; beyond that, a ready-mix delivery truck saves money and labor. Eighty-three 60 lb bags weigh nearly 5,000 lb — that is a lot of mixing by hand or even with a rented drum mixer.
A few concrete-specific things to watch:
- Sub-base variation. If your gravel base is uneven or the excavation dips, the actual pour depth will exceed the planned thickness in spots. Measure depth at the deepest point, not the average.
- Bag yield assumptions. The 0.45 and 0.60 cubic foot figures assume proper water-to-mix ratios. Adding too much water to make the concrete easier to work increases yield slightly but drops compressive strength significantly.
- Continuous pour. Concrete begins setting within about 45 minutes of mixing. Have enough help and equipment lined up before you open the first bag.
- Ordering ready-mix. Suppliers typically have a 1-yard minimum and charge a short-load fee under 3 yards. For this 1.23-yard slab, that fee may push ready-mix above the cost of bagged — but it eliminates 80+ bags of manual mixing.
- Odd shapes. For an L-shaped patio or circular pad, break the area into rectangles, calculate each separately, and add the volumes together. The calculator handles one rectangle at a time.
This calculator gives a volume estimate for planning your material purchase. For structural slabs, footings subject to building code, or load-bearing applications, follow an engineer's or local code official's specifications for thickness, reinforcement, and mix design.